Commercial and industrial robots are in widespread use for performing complex tasks. Robots perform tasks economically and precisely as compared to human beings. Further, robots are also employed to perform certain jobs that are dangerous, difficult, unsuitable and unsafe for human beings.
The structure of the robots includes kinematic chain having links, actuators and joints, each of which are configured to allow one or more degrees of freedom. Robot mechanisms with six degrees of freedom have been designed, and used in industry for accurately locating an object in a workspace. Conventional mechanisms in robots with six degrees of freedom use either a serial link mechanism or a parallel link mechanism to locate an object. The six degrees of freedom of any solid object include a position of the object's centre of gravity in an orthogonal three dimensional space denoted by x, y, and z co-ordinates, and the object's orientation about the centre of gravity denoted by three angles, often designated as yaw, pitch and roll. Position and orientation are often combined, and are collectively referred to as location.
Serial link mechanisms, which are generally open loop chains, provide for locating an object by means of an arm linkage for position, and a wrist linkage for the orientation. As each successive link in the serial chain is carried by its predecessor, the power required to move each link is compounded by the mass of the actuators powering its successors. Therefore, there is an increase in size and cost of all the actuators except for the last one in the chain. The additional power required does not contribute to the motion of the object being located by the robot. This results in a large ratio of the mass of the robot to the mass of the object or the payload that the robot can locate.
Parallel link mechanisms, which are generally closed loop chains, provide for locating an object by six arm links. In such mechanisms, all the actuators combine to contribute to the motion of the object. However, practically, the space within which the object can be located is limited due to interference of one link with a neighboring link. Parallel link robots have a smaller weight to payload ratio than serial link robots, and are used when their limited workspace is sufficient for the application.
Further, attempts have been made to build a manipulator employing a combination of serial link and parallel link mechanisms. One such manipulator is disclosed in a U.S. Pat. No. 6,047,610 granted to Stocco et al. (hereinafter US '610). FIG. 1 shows a structure of a robotic manipulator M having six degrees of freedom as disclosed by US '610. The manipulator M of US '610 includes first and second five-bar linkages 2 and 3 set on a rotatable base 4 and 5, respectively. Each of the five-bar linkages 2 and 3 are attached to a platform P via respective universal joints U2 and U3. Further, it is apparent from FIG. 1 that the links which form the five-bar linkage 2 are serially connected to each other by using serial link mechanism. Similarly, the links which form the five-bar linkage 3 are serially connected to each other by using serial link mechanism. Further, each of the five-bar linkages 2 and 3 is engaged with the platform P so as to form a closed loop chain or a parallel link.
It should be noted that a movement of each of the five-bar linkages 2 and 3 is limited due to interference of link 2 with link 3 and vice versa. Therefore, the amount of workspace maneuvered by the manipulator M, as disclosed in US '610, is substantially less and is identical to that of a parallel link robot.
Therefore, there is a need for a mechanism which enables a robot to maneuver substantially more amount of workspace than a parallel link robot and has a smaller weight to payload ratio than a serial link robot.